What does the term "parts per million" (ppm) commonly measure?

The term "parts per million" (ppm) is primarily used to measure the concentration of a substance in a solution. This measurement indicates how many units of a particular substance are present in one million units of the total solution. This method is particularly useful in various scientific fields, including environmental science, chemistry, and biology, as it allows for the comparison of very small concentrations, which are often encountered in chemical analyses, pollution assessments, and other applications where substances are present at low levels.

For example, if you are measuring the concentration of a contaminant in water, stating that it is present at a level of 5 ppm means that there are 5 parts of the contaminant for every million parts of water. This unit of measure helps in understanding the potential impact of substances on health and the environment.

The other choices do not accurately reflect the common use of the term ppm. Gas emissions might be measured in different units related to volume or mass rather than ppm. Temperature variations and the length of wastewater discharge are entirely different categories, as they pertain to physical measurements rather than concentration.